The pedalling of a bicycle requires a rotary motion in which force generated by the legs of the rider is transmitted to a pedal, and through a crank arm, to a sprocket. The sprocket engages a chain, which is also engaged to a second sprocket that is connected to the rear wheel. Rotary motion of the continuous chain drives the second sprocket and the rear wheel to propel the bicycle forward. This basic bicycle assembly has proven to be very effective, however, there are drawbacks to this design. The force transmitted by the legs to the pedal generates torque through the action of the force on the crank arm. This torque is at a minimum when the pedals are immediately under the rider, until the pedals have reached about the two o'clock position (or the ten o'clock position if the rotation is counterclockwise), because the force of the legs is being transmitted directly through the axis of rotation of the crank arms. The position immediately under the rider is referred to as Top Dead Center (TDC), a term which is well understood in the art. It is more difficult to begin or continue to propel a bicycle when the pedals are in this TDC position. Pedalling at the position of TDC, to about the two o'clock position (or the ten o'clock position if the rotation is counterclockwise), is rendered more difficult if added force is required to propel the bicycle, as when travelling up an incline. It would therefore be desirable to provide a bicycle which will transfer the effort of the rider more effectively to propel the bicycle.
Efforts to provide an increase in torque during pedalling rotation have been attempted, and have generally employed a mechanism or method which at some point in the rotation of the crank assembly extends the length of the crank arm, thereby increasing the radius of rotation. See, for example, U.S. Pat. Nos. 518,456; 595,535; 4,560,182; 563,821; 631,276; 4,816,009; and French Patent No. FR2,283,042. Each of the mechanisms described in the above-mentioned references, achieve their desired result by increasing the length of the crank arm during the rotation cycle. This approach is different than the subject invention, which increases torque and effects a mechanical advantage on the crank arm without extending the length of the crank arm during the rotation cycle.
Another French Patent (No. FR 2,540,455) describes a pedal drive for a bicycle having cranks which are curved back on themselves by a short extension arm. The object stated for the mechanism described in the FR '455 patent is to provide an area, between the axis of the pedal sprocket and that of the foot rest pedal), a greater radius. Thus, the increased torque achieved by the crank arm of the FR '455 patent is effectively addressed by extending the length of the crank arm. Although the FR '455 patent claims a greater crank leverage, the short extension arms described and shown in the FR '455 patent are not of sufficient length to obviate the long-standing and well-recognized problem of a dead spot in the pedaling rotation, Chaumer, D. and M. Halstead (1990) THE TOUR DE FRANCE COMPLETE BOOK OF CYCLING; and LeMond, G. and K. Gordis (1987) GREG LEMOND'S COMPLETE BOOK OF CYCLING; see also, Burke, E. (1986) SCIENCE OF CYCLING (discussing the important parts of the pedaling cycles). This dead spot occurs when the pedal is in the position of TDC or within about 60 degrees of TDC (the 10 o'clock to 2 o'clock position). Accordingly, the device of the FR '455 patent does not solve the long-standing problem of eliminating this dead spot at between TDC and 60 degrees.